A dark fiber network or simply dark network[1] is a privately operated optical fiber network that is run directly by its operator over dark fiber leased or purchased from another supplier, rather than by purchasing bandwidth or leased line capacity. Dark fiber networks may be used for private networking, or as Internet access or Internet infrastructure networking.

Because there is no intermediate resale of capacity, dark fibre networks can operate using the latest optical protocols using wavelength division multiplexing to add capacity where needed, and to provide an upgrade path between technologies without removing the network from service.

They offer very high price-performance for network users who require high performance, such as Google, which has dark network capacities for video and search data,[1] or wish to operate their own network for security or other commercial reasons.

However, dark fiber networks are generally only available in high-population-density areas where fiber has already been laid, as the civil engineering costs of installing fiber to new locations is often prohibitive. For these reasons, dark fiber networks are typically run between data centers and other places with existing fiber infrastructure.